Nuclear reactors are used today in numerous ways, e.g., to generate electric power and for special tests. Nuclear power plants employ a nuclear reactor. The fuel is a radioactive material which undergoes a fission reaction. Nuclear fission is the splitting of the atomic nucleus, usually into two or three larger particles, two or more neutrons, and other smaller particles with the release of a relatively large amount of energy. The average amount of energy released in the various fission reactions is about 200 M.sub.e V (million electron volts), which is dramatically higher than the energy produced from a chemical reaction such as oxidation which is used in fossil-fuel plants when coal or oil is burned.
During nuclear fission in energy production, an exceedingly large amount of energy is released and excess neutrons are produced which permits a chain reaction. These factors make it possible to design nuclear reactors which are self-sustaining, i.e., wherein reactions occur with the continuous release of energy.
Some current light-water cooled nuclear power plants utilize uranium oxide as a fuel. Solid cylindrical rods, instead of plates, are the most common shape for the fuel. For most reactor applications, these fuel rods are protected by a cladding of aluminum, stainless steel, zirconium or zirconium alloy and are assembled into a unit. Other typical constituents of reactors include aluminum-clad uranium metal for plutonium production reactors, stainless steel-clad UO.sub.2 dispersed in stainless steel for propulsion reactors, and zirconium or stainless steel-clad UO.sub.2 pellets for central station power reactors.
Zirconium is used in nuclear technology because of its properties which include insolubility in water and cold acids, corrosion resistance, low neutron absorption, and low toxicity. In addition to these desirable properties, zirconium also exhibits good strengths at high temperatures, corrosion resistance to high velocity coolants, avoidance of formation of highly radioactive isotopes, and resistance to mechanical damage from neutron radiation. An important application for zirconium is as the base metal in an alloy known as Zircaloy II, comprised of 1.5% tin, 0.35% iron-chromium-nickel, 0.15% oxygen, and the balance zirconium. This alloy is widely used in water cooled nuclear reactors because of its excellent corrosion resistance up to about 350 degrees in H.sub.2 O, and its low neutron cross-section. The term "Zircaloy" is a trademark for alloys of zirconium and nickel which are used as cladding for nuclear fuel elements and for other reactor applications. "Zircaloy II" is a particular Zircaloy.
Even though corrosion-resistant alloys are used to clad the fuel, radiation damage and corrosion can still occur to these alloys. Corrosion is usually the destruction, degradation or deterioration of material due to the reaction between the material and its environment. In a restricted sense corrosion consists of the slow chemical and electrochemical reactions between a metal and its environment; in a broader sense corrosion is the slow destruction of materials by chemical agents and electrochemical reactions.
The severe environment in the core of a boiling water nuclear reactor can include temperatures of 290.degree. C. or greater, pressures of 1,000 psi or greater, and radiation of 10.sup.9 rads per hour gamma and 10.sup.13 rads per hour neutron. The Zircaloy alloys are among the best corrosion resistant materials when tested in water at reactor operating temperatures, about 290.degree. C., but without exposure to nuclear radiation. The corrosion rate under these conditions is very low and the corrosion product is a uniform, tightly adherent, black ZrO.sub.2 film. In actual service, however, the Zircaloy is irradiated and is also exposed to radiolysis products present in reactor water. The corrosion resistance properties of Zircaloy deteriorate under these conditions and the corrosion rate thereof is accelerated.
The deterioration under actual reactor conditions of the corrosion resistance properties of Zircaloy is not manifested in merely an increased uniform rate of corrosion. Rather, in addition to the black ZrO.sub.2 layer formed, a localized, or nodular corrosion phenomenon has been observed especially in boiling water reactors. In addition to producing an accelerated rate of corrosion, the corrosion product of the nodular corrosion reaction is a highly undesirable white ZrO.sub.2 bloom which is less adherent and lower in density than the uniform corrosion product of black ZrO.sub.2.
The increased rate of corrosion caused by the nodular corrosion reaction will be likely to shorten the service life of the tube cladding, and also this nodular corrosion will have a detrimental effect on the efficient operation of the reactor. The white ZrO.sub.2, being less adherent, may be prone to spalling or flaking away from the tube into the reactor water. On the other hand, if the nodular corrosion product does not spall away, a decrease in heat transfer efficiency through the tube into the water is created when the nodular corrosion proliferates and the less dense white ZrO.sub.2 covers all or a large portion of a tube.
It is, therefore, desirable to be able to detect or monitor the corrosion, e.g., nodular corrosion or general corrosion, to the fuel cladding and other members in a reactor core.
It is, therefore, an object of this invention to provide an apparatus and method of monitoring corrosion to members such as nuclear fuel cladding in the core of a nuclear reactor. It is another object of this invention to provide such an apparatus and method for monitoring of corrosion to such members by measuring potential changes in a sensor located in the nuclear reactor core. Furthermore, it is an object of this invention to monitor corrosion on a scored surface area on the sensor, e.g., to monitor nodular corrosion that is promoted by such scoring. It is also an object of this invention to provide such an apparatus and a method employing the apparatus wherein the sensor is comprised preferably of zirconium, and even more preferably of a zirconium alloy or "Zircaloy" alloy. It is even further preferred that the zirconium, zirconium alloy or Zircaloy alloy be from the same lot as that used to form, for example, the fuel cladding.